I have a 10-gallon Rubbermad mash tun and a 7.5-gallon stainless kettle. The kettle is really too small for full 7-gallon boils, and getting the mash temp set and stabilized is a bit of a Rube Goldberg enterprise with a heatstick.

So I'm looking to build an electric keggle out of a 1/2-barrel keg, and hoping there would be a way to use it for the heat source throughout all three steps - heating strike and sparge water, maintaining mash temp (and allowing step mashes) and boiling.

I have a Ranco controller and plenty of wiring and plumbing experience. I have a copper immersion chiller that I use post-boil. I currently collect 1st mash runnings in 6-gallon buckets, and would continue to do so, since the "boil keggle" would not be freed up until sparging is complete.

Is there a 240V electric element size that would be suitable for both heating strike water and boiling? Is is the energy density requirement too different?

Could I somehow leave my immersion chiller in the keggle full-time and use it was a HERMS during mashing, and for chilling post-boil?

I am actually in the middle of finalizing a system that is the same as you described. I built a 15 gal. aluminum kettle with a 5500W 240V Camco ripple element and a toolbox control panel with all Auber parts with a 12 gallon rectangular cooler. With a PID the 5500W element works perfect. It brings 10 gallons of water from about 75 to a boil (with the lid off and and losses through the aluminum) in about an hour. And when the boil is reached I just put it into manual mode and set it at 75% output to minimize my boil-off rate.
I average about 3 degrees F of heating per minute at full power.

I plan to have a two tier system in the kitchen with the boil kettle higher than the mash tun. I will bring the sparge water to temp and then gravity feed into the mash tun. Next I will fly sparge while draining the runnings into a bottling bucket (not the one I actually use for bottling because I have read that there may be some bacteria present even after the mash). And after I have drained all of my sparge water I will transfer the wort to the boil kettle up top. I will most likely pour it for the first few batches until I decide to spend even more money on a pump. Then I will boil, cool (might need to consider a plate or counter flow chiller due to the IC and the heating element interfering), and then gravity drain to the fermenter.

This setup will allow for both fly sparging or batch sparging with 1 pump and 1 electric kettle, and with a pump and recircing you can do stepped mashes as well by adding a calculated amout of hot water. You also will not be pulling directly from the mash tun if you have a pump, so this might help prevent stuck sparges.

You could pump wort from the mash tun through the IC which could rest in the boil kettle as a sort of HERMS coil I guess. Might need to set the temp set point a degree or two higher to account for losses.

Disclaimer: My first brew like this (sans pump) is not until next week so there may be flaws in my reasoning.

I am actually in the middle of finalizing a system that is the same as you described. I built a 15 gal. aluminum kettle with a 5500W 240V Camco ripple element and a toolbox control panel with all Auber parts with a 12 gallon rectangular cooler. With a PID the 5500W element works perfect. It brings 10 gallons of water from about 75 to a boil (with the lid off and and losses through the aluminum) in about an hour. And when the boil is reached I just put it into manual mode and set it at 75% output to minimize my boil-off rate.
I average about 3 degrees F of heating per minute at full power.

I plan to have a two tier system in the kitchen with the boil kettle higher than the mash tun. I will bring the sparge water to temp and then gravity feed into the mash tun. Next I will fly sparge while draining the runnings into a bottling bucket (not the one I actually use for bottling because I have read that there may be some bacteria present even after the mash). And after I have drained all of my sparge water I will transfer the wort to the boil kettle up top. I will most likely pour it for the first few batches until I decide to spend even more money on a pump. Then I will boil, cool (might need to consider a plate or counter flow chiller due to the IC and the heating element interfering), and then gravity drain to the fermenter.

This setup will allow for both fly sparging or batch sparging with 1 pump and 1 electric kettle, and with a pump and recircing you can do stepped mashes as well by adding a calculated amout of hot water. You also will not be pulling directly from the mash tun if you have a pump, so this might help prevent stuck sparges.

You could pump wort from the mash tun through the IC which could rest in the boil kettle as a sort of HERMS coil I guess. Might need to set the temp set point a degree or two higher to account for losses.

Disclaimer: My first brew like this (sans pump) is not until next week so there may be flaws in my reasoning.

That sounds like a good system. So if you elect to batch-sparge, will you recirculate the wort at all-or just let it sit for the full hour? I realize my wort needs to be clearer and I know circulating it through the grain bed should help.

Where are you measuring temperatures? A single thermowell near the bottom of the kettle, and bottom of the mash tun? My current system allows a significant temperature gradient throughout the strike water and mash be, since they are stagnant.

I am also concerned about the immersion chiller interfering with the heating element. Let me know if you solve that one. Also, is there any risk of scorching the wort or sugar additions on the bottom of the kettle since you can't stir around the element?

I was going to vorlauf the fist gallon or so going into the bucket to help clarify. I guess for both fly and batch sparging that the wort would sit for 45min to an hour so I am not sure of the effect this will have. I could install a small 1000w heating element just to bring it to 170 for mashout. To be honest I don't know what problems are associated with letting the wort sit prior to boiling.

In my system I can stir around the element without problems but for 5 gallon batches there is only 3 inches of water over the element. I found that I can tuck that extended single loop of the IC under the element to help get better contact with the water. I plan to switch to 10 gallon batches in the future and this will no longer be a problem.

I placed my RTD near my side pickup in the kettle. When I hit strike water temps I will just give the pot a good stir and wait another 5 min for the temp to balance out and then transfer to the mashtun. As for the temps in the mash tun I was thinking of installing a long probe through the side. Since it is a cooler I wasn't expecting too much of a difference after doughing in and closing the lid.

I was going to vorlauf the fist gallon or so going into the bucket to help clarify. I guess for both fly and batch sparging that the wort would sit for 45min to an hour so I am not sure of the effect this will have. I could install a small 1000w heating element just to bring it to 170 for mashout. To be honest I don't know what problems are associated with letting the wort sit prior to boiling.

In my system I can stir around the element without problems but for 5 gallon batches there is only 3 inches of water over the element. I found that I can tuck that extended single loop of the IC under the element to help get better contact with the water. I plan to switch to 10 gallon batches in the future and this will no longer be a problem.

I placed my RTD near my side pickup in the kettle. When I hit strike water temps I will just give the pot a good stir and wait another 5 min for the temp to balance out and then transfer to the mashtun. As for the temps in the mash tun I was thinking of installing a long probe through the side. Since it is a cooler I wasn't expecting too much of a difference after doughing in and closing the lid.

yeah my cooler holds temp for 60 mins pretty well - I just want to have the option to undershoot the initial strike then "sneak up" on the target temperature. And also to do a multi-step mash.

Surprised that is only 3" of water over your element, but I guess your kettle has a fatter form factor than a keg so it makes sense for 10 gallon matches.

Can I reuse my existing RANCO controller for controlling the HERMS pump or BK element? It is a simple on/off, not PID so I may be limited but I was hoping to only buy one pump plus a single new controller.